Legend has it that Isolt Sayre (b. 1603, co-founder of  Ilvermorny School of Witchcraft and Wizardry) is the witch that brought sticky toffee pudding from the muggle world into the wizarding world. Isolt’s mother, Rionach Sayre, was born into the House of Gaunt, which was a pure-blood family that descended from Salazar Slytherin. Rionach, and her husband William, rejected her family's beliefs about pure blood superiority and were friendly with muggles. In 1608, dark wizards set Isolt’s house on fire, killing both her parents. Isolt, who escaped the fire, was sent to be raised by her mother’s estranged sister, Gormlaith Gaunt. Gaunt was a staunch believer in the plans of her ancestor Salazar Slytherin, but Isolt remembered the friendly ways of her parents and was angry at her aunt after learning that she was behind the fire and the death of her parents. Isolt eventually ran away, cutting her hair short and pretending to be a muggle boy named "Elias Story" (an anagram of "Isolt Sayre.”) With her aunt trying to track her, she sailed away to America on the Mayflower in 1620. In 1714, near her death (she lived to be well over 100 years), she took a voyage back to Ireland to see her homeland. While there, she met a muggle woman named Fustain who, when she was a young girl, was a cook for Lord Piaras Feiritéar. While working for him, Fustain became exceptionally known for her skill at making puddings. She expressed to Isolt  how distraught she was that Christmas pudding (then called Christmas Frumenty ) was banned by Puritans (in 1664.) Feeling sorry for her friend, Isolt paid a visit to King George I and convinced him to re-establish it as part of the traditional Christmas meal. Elated, Fustain made Isolt a pudding she claimed to have invented called Sweet Butter Porridge which, today, is known as sticky toffee pudding.

Sources: harrypotter.wikia.com, telegraph.co.uk, whychristmas.com, foodtimeline.org, woodycandy.com, A New History of Ireland, volume one, Oxford, 2005